(i) Field of the Invention
The present invention relates to an apparatus for cutting a plate such as a corrugated cardboard or a steel plate into a predetermined size, and more particularly to an apparatus for cutting the plate with improved cutting accuracy and high-speed cutting performance.
(ii) Description of the Prior Art
FIG. 2 shows a simple configuration of an apparatus for cutting corrugated cardboard into a predetermined size. In FIG. 2, numeral 1 denotes corrugated cardboard which moves in a direction shown by an arrow. Numerals 2 denote knife cylinders which rotate in the directions indicated by the arrows so that edges of upper and lower knives are brought together on the cardboard 1 to cut the cardboard 1. Numeral 3 denotes a wheel for measuring a moving speed of the cardboard 1. Numeral 4 denotes a cardboard moving speed detector which generates a speed signal in response to a rotation of the wheel 3. Numeral 5 denotes a rotational angle detector for measuring a rotational movement of the knife cylinders 2. Numeral 6 denotes reduction gears for coupling the knife cylinders 2 and a motor 7. Numeral 8 denotes a motor speed detector for measuring a rotating speed of the motor 7. Numeral 11 denotes a cut length setting unit of the cardboard. Numeral 10 denotes a motor speed command signal generator which is supplied with a cutting command from the cut length setting unit 11, a cardboard moving speed signal from the cardboard moving speed detector 4 and a knife cylinder rotational angle signal from the knife cylinder rotational angle detector 5 and generates a rotating speed command signal for the motor 7. Numeral 9 denotes a speed control device which generates a motor applied voltage so that the rotating speed command signal from the motor speed command generator 10 is equal to a speed detecting signal from the speed motor detector 8. The speed control device 9, the motor 7 and the motor speed detector 8 form a knife cylinder driver.
FIG. 3 shows a configuration of the driver. In FIG. 3, numeral 91 denotes a speed compensator responsive to a difference between the rotating speed command signal from the motor speed command signal generator 10 and the speed detecting signal from the motor speed detector 8 to produce a torque command signal so that there is no difference. Numeral 92 denotes a torque compensator which is responsive to a difference between the torque command signal from the speed compensator 91 and a torque detecting signal from a motor torque detector 94 to produce a motor armature voltage command signal by a propotional-plus-integral control (referred to as PI control) or a proportional-plus-integral-plus-derivative control (referred to as PID control) so that there is no difference. Numeral 93 denotes a power amplifier composed of a thyristor which amplifies the motor armature voltage command signal from the torque compensator 92 and produces the motor applied voltage.
Such a prior art apparatus requires electric power of several tens kW or more as electric power of driving the knife cylinders. More particularly, when the speed control of the motor 7 is attained by the power amplifier 93 in order to process large electric power, the phase angle control using a thyristor is required. In this case, while the phase angle control using the thyristor is generally used in application requiring high speed response, the frequency response of the phase angle control is limited by a frequency of the power source. Accordingly, the frequency response is deteriorated in the phase control of the AC power source. For example, the frequency response band is about 15 Hz in the phase angle control using a three phase full-wave rectification and higher responsive characteristic can not be attained. Accordingly, there is a limitation in cutting accuracy and high speed cutting performance.
In order to enhance the speed response of the knife cylinders 2, it is necessary to use a motor having a special structure in which the inertia of the armature of the motor 7 is reduced.